Age and geochemical characteristics of Malasongduo Formation rhyolite in Riza Mountain, east Tibet, and its geological significance

In order to determine the formation age and tectonic environment of east Tibetan Malasongduo Formation rhyolite and provide new evidence for studying ancient Tethys closing time, the authors applied the LA-ICP-MS method for the precise zircon U-Pb dating and studied petrology and geochemistry. The results show that the weighted average ²⁰⁶Pb/²³⁸U age of rhyolite magma in this area is 244 ±1.2Ma (MSWD=0.59), which accurately defines the formation age of the Malasongduo Formation rhyolite, i.e., Early-Middle Triassic. Petrographic and geochemical studies show that Malasongduo Formation rhyolite has high silica (SiO₂=72.72%~76.88%) and alkali (ALK (K₂O+Na₂O)=6.64%~7.41%, K₂O>Na₂O), and exhibits peraluminous nature (Al₂O₃=11.76%~13.03%, A/CNK=1.17~1.31); the rocks are enriched in large ion lithophile elements K and Rb, and high field strength elements Th and U, but depleted in large ion lithophile elements Sr and Ba, and high field strength elements such as Nb, Ta, P, Zr, Hf and Ti; REE patterns show that LREE is enriched (LREE/HREE=1.93~2.89), light rare earth elements (LREE) is slightly higher than the degree of fractionation of heavy rare earth elements (HREE), and there exist right-inclined V-shaped distribution and obvious negative Eu anomalies (Eu=0.36~0.41). The genesis of the rocks may be related to the underplating of the mantle derived magma and the thinning of the thickening crust. They were formed by the mantle derived basic magma underplating resulting in dehydration of crustal material and partial melting. They were mainly formed by partial melting of continental crust and aluminosilicate source rocks, and the magma underwent crystallization differentiation during the process of rising. The chemical properties have the evolution trend of S type to A type rhyolite, and have the dual characteristics of the syn-collisional arc volcanic rocks and the post collisional A type rhyolite. According to the comprehensive study, the study area was in a short post orogenic extensional tectonic environment on the post collisional active continental margin in the Early-Middle Triassic period, and the ancient Tethys Ocean (Jinshajiang Ocean) had been closed before that.